Tracer Techniques for Thickness of Very Thin Vacuum-Evaporated Metal Films

The vacuum evaporation process has attained importance in the field of optical coatings, sensitization methods, printed circuitry, and similar applications. Some problems exist in the determination of condensed metal film thickness, evaporant spatial dispersion, condensand profiles, and composition. A number of assay methods, such as the microbalance system, light absorption, beta scatter, and others, may be used for measurement of condensed films which vary in thickness from less than the mono-layer to over several hundred Ångstroms. Conventional methods are not sufficiently sensitive for exceedingly thin films, but the direct tracer scheme with the tagged radioactive metal charge exhibits an experimental reach sufficient to assay even the most tenuous sensitization layers.

Determinations of dispersion patterns, condensand surface concentration, condensand profile and uniformity, binary evaporant assay, trace contaminations, and alloying may be made by the tracer scheme. Techniques and the advantages and disadvantages of the method are discussed.

This paper will be concerned exclusively with the determination of surface concentrations of the more tenuous thin films-those metal layers which fall about, or below, 100 Å in thickness. Although the technique is also applicable to films of thicknesses greater than 100 Å, the thicker condensates are accurately measurable by sundry well-established systems (1,2).2 Discussion of the direct tracer scheme will be here limited principally to those films below the 100-Å region and, in particular, to its application to the condensand at the mono-layer and even below. It is in this range that the direct tracer technique may have some useful application which will supplement and compete efficiently with the various other methods for assay of thin layers.

The method will be described principally in connection with metallic layers condensed from the evaporant produced by the vacuum distillation of certain metal charges, although the direct system is not inapplicable to other tagged layers. It is safe to predict that as the direct system of test is pursued, the distilland condensate assay will become a minor application of this test mechanism.

The incorporation of the tagged element into a film to determine surface concentration is not a unique idea (3). This, in essence, could be done each time a dry planchet is assayed. Nevertheless, the method has a distinct applicability to the vacuum distillation phenomena and to the very thin condensands associated with the evaporation of metal charge.